1. Field of the Invention
This invention relates to power amplifiers and more particularly to a snubber circuit for operation of a power amplifier with reactive loads.
2. Description of the Prior Art
In the design of transistor switching bridge amplifiers in inverter power supply circuits, the designer must protect the transistors from breakdown due to inductive turn-off when the current lags the load voltage. Load current flows through the transistor during the turn-off transition and goes to zero only after the transistor voltage has risen to the full supply voltage or higher such as due to inductor transients. Without careful design and compensation, transistors are not capable of turning off with inductive load currents. Snubber circuits that are attached to the output of a power amplifier function to absorb load current during turn-off.
One example of a prior art snubber circuit is shown in FIG. 1 which comprises a diode and capacitor in series across a switching transistor. A resistor and inductor coupled in series are placed across the diode for dissipating charge built-up across the capacitor in the course of absorbing load current. The resistor dissipates considerable power in discharging the capacitor. In addition, the snubber circuit shown in FIG. 1 imposes stress on the switching transistor at turn-on due to the addition of reset current.
In U.S. Pat. No. 3,189,796, entitled "Apparatus for Suppressing Transient During Switching" by Lynn L. Tipton which issued on June 15, 1965, the problem of a transistor switching a reactive load was addressed. The patent describes a transformer device having a magnetic core and first and second windings with the core being proportioned to saturate within the operating range of the current flowing in the windings.
In U.S. Pat. No. 3,681,659, entitled "Amplifier Protective Circuit" by Tadao Suzuki, which issued on Mar. 26, 1971, a protective circuit is described which positively and effectively limits the current flowing in the transistors for their protection. The power dissipation of the transistors to be protected and impedance of the load are sensed by the protective circuit.
It is therefore desirable to provide a power amplifier with a snubber circuit that eliminates the need for resistors to dissipate the turn-off switching energy.
It is further desirable to provide a snubber circuit that returns to the amplifier power source the amount of energy that would have been dissipated either in the switching transistors or in the resistors of a conventional snubber circuit.
It is further desirable to provide a snubber circuit which can be activated or deactivated by low level logic so that no turn-on stress is added to the amplifier transistors at times when the snubber circuit is deactivated.